The present invention relates to a piston ring for non-circular, particularly oval or elliptical cylinders, with the ring being interrupted at one point of its circumference. As seen in the circumferential direction, radii of curvature of the ring deviate from one another before and after installation of the ring in an associated cylinder.
U.S. Pat. No. 4,306,730, issued Dec. 22nd, 1981, to Honda et al, discloses an elliptical piston ring having a gap provided in a short region of the ring. The ring is widened in its long region before installation in an engine cylinder, so that, in the installed, cold state, when compressed again to its gap tolerance, the radial pressure exerted onto the surrounding cylinder wall is uniform over the entire circumference of the ring. This is accomplished by a change in the radii of curvature of the ring by mechanical deformation of the ring before and during assembly thereof.
It is known that piston rings must be fit tightly into a cylinder so that no light passes through a gap between the running surface of the piston ring and the cylinder wall. Therefore, in their non-installed state, the contour of piston rings must be determined to within at least 0.01 mm. In the piston rings discussed in the above-mentioned U.S. patent, the contour of the rings is found empirically. With the degree of precision required, however, an empirical contour determination for a piston ring in the non-installed state with simultaneous conclusions as to the radial contact pressure involved in the installed state appears to be highly dubious.
Moreover, the piston ring in question also has the following drawbacks:
During engine operation, oval piston rings, and elliptical rings in particular, expand more in the longitudinal direction thereof than does the surrounding cylinder wall. At low temperatures, which accompany the installed but not-operating mode, a certain point of the piston ring will contact the cylinder wall at another point than it will contact the same wall in the warm state encountered during the operating mode. Piston ring and cylinder wall thus change their relative positions as a function of temperature. For a piston ring as disclosed in U.S. Pat. No. 4,306,730, this results in an increase of the radial contact pressure in certain circumferential regions which ultimately produces increased wear with the result of a short service life. Depending on the contour of the piston ring, the thermally effected change in position may also produce such a deformation in the transition region of the radii of curvature that the ring will be untight to light; i.e., it will permit the fluid medium to be sealed off to pass through a gap between the piston ring running surface and the cylinder wall.
With round cylinders and their associated piston rings, thermal expansion of the rings does not produce problems because the radius of curvature of the rings is constant over the circumference thereof. Thus, the change in length need be considered only in the amount of play between the gap ends. Calculations for thermal expansion of round piston rings are known in the art. See, for example, German Published Application 1,021,666, or German Pat. No. 842,742. But, the statements made in these publications cannot be transferred to any piston rings which deviate from the circular shape, and particularly not to piston rings of elliptical or other oval contours.